1. Field to which invention relates
The invention relates to a high pressure mixing head for the production of a chemically reactive mixture of at least two plastics components and other mixtures comprising a mixing chamber, which has inlet ports for the individual plastics components and an outlet port for the plastics component mixture, and an expulsion piston, which is arranged in the mixing chamber and has the same cross-section as it, and which is connected with a drive piston, arranged in a drive cylinder and adapted to be actuated by pressure medium, in particular hydraulic oil, for movement of the expulsion piston in a reciprocating manner out of a rear position leaving the inlet ports open into a position adjacent to the front outlet port, the inlet ports being simultaneously shut off from the mixing chamber.
2. The prior art
In the case of a previous mixing head of this type the inlet ports for the individual plastics components open and close alternately and simultaneously without separate control means, having to be provided for the opening and closing, since during the reciprocating movement of the expulsion piston it sweeps over the inlet ports. The expulsion of the mixture is further accompanied by purging from the mixing chamber wall to remove vestiges of mixture, which are removed by the expulsion piston, having the same cross-section as the mixing chamber as the latter is driven outwards. For return of the plastics components the expulsion piston is provided with transfer flow ducts by means of which the inlet ports are arranged to be connected with the return flow ducts with which they are provided, at the same time as they are shut off from the mixing chamber.
In the case of this previous high pressure mixing head with counter-flow injection mixing the expulsion piston is moved during the whole stroke of its reciprocating movement with the same or with a constant velocity. This velocity of the expulsion piston is however limited by the constructional complexity, which can be tolerated for the hydraulic drive required for the piston.
In certain cases it has been found in practice that with certain plastics components, more particularly in the case of plastics components which are difficult to mix up, there is only an incomplete mixing action at the beginning and at the end of the injection operation. The reason for this insufficient mixing, which only occurs in the beginning and at the end of the injection operation, is most probably due to the fact that at the beginning and at the end of the mixing operation when the expulsion piston is sweeping over the mixing chamber inlet ports only a part of the full entry cross-section of the ports is available for a short time so that the injection operation differs in character from normal injection with the full entry cross-section available.